U.S. patent number 4,507,364 [Application Number 06/502,662] was granted by the patent office on 1985-03-26 for perpendicular magnetic recording medium.
This patent grant is currently assigned to Teijin Limited. Invention is credited to Sadao Kadokura, Masahiko Naoe, Takashi Tomie.
United States Patent |
4,507,364 |
Kadokura , et al. |
March 26, 1985 |
Perpendicular magnetic recording medium
Abstract
Magnetic recording medium conventionally utilizes the in-plane
magnetization mode, but, recently, the perpendicular magnetization
mode utilizing the perpendicular anisotropy of an hcp cobalt alloy
layer, in which C axis is oriented normal to the layer surface, is
proposed. The known perpendicular magnetic recording medium has
been produced by an RF sputtering, but such medium is of too low
flexibility to use it in the form of a magnetic tape. In addition,
the production rate of the perpendicular magnetic recording medium
by RF sputtering is very low. The perpendicular magnetic recording
medium is very flexible due to particle pattern (FIGS. 10, 12 and
13) completely distinct from the conventional columnar pattern
(FIGS. 8 and 11). In addition, the production rate is high, because
the base (20) is located beside a space between the targets
(T.sub.1, T.sub.2) of a sputtering device and further the magnetic
field is generated perpendicularly to the sputtering surfaces
(T.sub.1s, T.sub.2s) by a field coil (32) or magnets (32, 33). The
present invention makes it possible to commercially produce the
perpendicular magnetic recording medium, especially in the tape
form.
Inventors: |
Kadokura; Sadao (Hachioji,
JP), Tomie; Takashi (Hino, JP), Naoe;
Masahiko (Tokyo, JP) |
Assignee: |
Teijin Limited (Osaka,
JP)
|
Family
ID: |
15982810 |
Appl.
No.: |
06/502,662 |
Filed: |
July 14, 1983 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
329822 |
Dec 11, 1981 |
4407894 |
Oct 4, 1983 |
|
|
Foreign Application Priority Data
|
|
|
|
|
Dec 12, 1980 [JP] |
|
|
55-174680 |
|
Current U.S.
Class: |
428/457;
428/836.1; 428/900; G9B/5.236; G9B/5.304 |
Current CPC
Class: |
G11B
5/64 (20130101); G11B 5/851 (20130101); Y10T
428/31678 (20150401); Y10S 428/90 (20130101) |
Current International
Class: |
G11B
5/64 (20060101); G11B 5/84 (20060101); G11B
5/851 (20060101); H01F 010/02 () |
Field of
Search: |
;428/694,457,900
;427/127-132,48 |
Other References
Iwasaki et al, IEEE Transactions on Magnetics, vol. MAG 15, No. 6,
11-79, pp. 1456-1458. .
Iwasaki, IEEE Transactions on Magnetics, vol. MAG 16, No. 1, 1-80,
pp. 71-76. .
Iwasaki et al, IEEE Transactions on Magnetics, vol. MAG-14, No. 5,
7-78, pp. 849-851..
|
Primary Examiner: Pianalto; Bernard D.
Attorney, Agent or Firm: Burgess, Ryan & Wayne
Parent Case Text
This is division of application Ser. No. 329,822, filed Dec. 11,
1981 and issued on Oct. 4, 1983 as U.S. Pat. No. 4,407,894.
Claims
We claim:
1. A perpendicular magnetic recording medium formed on a base and
comprising an hcp cobalt alloy layer having opposed major surfaces
and comprising mainly cobalt and additionally chromium, said layer
having a direction of easy magnetization in a direction normal to
the base, characterized in that the cobalt alloy layer is composed
of particles, with a boundary between the particles forming a
non-columnar pattern extending between said surfaces of said cobalt
alloy layer, as seen in a cross section of the layer observed with
a scanning electron microscope, thereby providing said cobalt alloy
layer with a homogeneous cross sectional structure.
2. A perpendicular magnetic recording medium according to claim 1,
wherein said base is a polyester film.
3. A perpendicular magnetic recording medium according to claim 1,
further comprising a layer of soft magnetic metal.
4. A perpendicular magnetic recording medium according to claim 2
or 3, wherein the degree of curl (Kp), which is expressed by:
##EQU2## is not more than 15%, the symbol "l.sub.0 " indicating the
length of a specimen of said medium without curl and the symbols
"h.sub.1 " and "h.sub.2 " indicating the deviation of a specimen
with curl for both ends of the specimen without curl.
5. A perpendicular magnetic recording medium according to claim 1,
wherein said particle pattern substantially conforms to FIG. 10 of
the accompanying drawing.
6. A perpendicular magnetic recording medium according to claim 1,
wherein said particle pattern substantially conforms to FIG. 12 of
the accompanying drawing.
7. A perpendicular magnetic recording medium according to claim 1,
wherein said particle pattern substantially conforms to FIG. 13 of
the accompanying drawing.
Description
The present invention relates to a perpendicular magnetic recording
medium of a cobalt alloy which comprises mainly cobalt and
additionally chromium and a method for producing the same. More
particularly, the present invention relates to a perpendicular
magnetic recording medium having a novel pattern as seen through an
electron microscope and to a method for producing the same. In
addition, the present invention also relates to an improved
sputtering device.
The present magnetic recording system fundamentally uses the
longitudinal (in-plane) magnetization mode, that is, a
magnetization being parallel to the base, to which the cobalt alloy
layer is applied.
Iwasaki has proposed in IEEE Transactions on Magnetics, Vol.
MAG-16, No. 1 January 1980, pp 71-76 a perpendicular magnetic
recording system which theoretically makes it possible to produce a
higher density recording than in the case of the longitudinal
magnetization mode. In the perpendicular magnetic recording system,
the magnetization perpendicular to the surface of the magnetic
recording layer is used for the recording. Research, for example as
shown in Japanese Laid Open Patent Application No. 52-134706 and
Technical Reports MR80-43 and MR81-5 of the Institute of
Electronics and Communication Engineers of Japan, has been
energetically carried out in an attempt to commercially apply the
perpendicular magnetic recording system and to elucidate the
properties of a magnetic medium required for the magnetic recording
devices.
The results elucidated by previous research regarding the
properties of a magnetic layer necessary for perpendicular magnetic
recording are now explained. The magnetic layer adapted to the
perpendicular magnetization system should be an alloy layer mainly
consisting of cobalt and additionally chromium and should have a
magnetic anisotropy perpendicular to the layer surface. This
magnetic anisotropy, i.e. perpendicular magnetic anisotropy, should
usually have a relationship of Hk.gtoreq.4Ms, wherein Hk and
4.pi.Ms are the anisotropy field and the maximum demagnetizing
field of a magnetic layer, respectively. This relationship
designates that the magnetic layer possesses a satisfactorily high
perpendicular anisotropy.
The perpendicular anisotropy may not have a relationship of
Hk.gtoreq.4.pi.Ms at any point on the magnetic layer if a
particularly high density magnetic recording is to be achieved.
Instead, a high saturation magnetization (Ms) is desired and,
therefore, chromium is incorporated into cobalt in such an amount
that a saturation magnetization (Ms) ranging from 200 to 800 emu/cc
is ensured. The cobalt, which is the major component of the
magnetic film, has an hcp (hexagonal closed packing) structure and
a uniaxial magnetic anisotropy which makes possible a high
anisotropy field. Such an anisotropy field is one of the properties
necessary for perpendicular magnetic recording. A cobalt alloy
layer, which has an hcp structure and a high perpendicular
orientation to the layer surface (C-axis of the alloy is
perpendicular to the layer surface), exhibits a satisfactorily high
anisotropy field Hk. The perpendicular orientation mentioned above
is evaluated by subjecting a magnetic film to X-ray diffraction,
obtaining the rocking curve of the diffraction peak from the (002)
plane of the hcp structure, and measuring the half value width or
dispersion angle .theta..sub.50 of the rocking curve. A half value
width .theta..sub.50 of 15.degree. or less is alleged to be
sufficient for excellent perpendicular anisotropy. The coercive
force H.sub.cv in the perpendicular direction, which is more than
100 Oe (Oersted), is allegedly sufficient for an excellent
perpendicular orientation.
It is reported in the Japanese Journal of Applied Physics Vol. 20,
No. 7 and in the Technical Report of the Institute of Electronics
and Communication Engineers of Japan MR81-5 that the above
properties necessary for the perpendicular magnetic recording mode
can be produced by a cobalt alloy layer in which from 15 to 25
atomic % of chromium is incorporated and that columnar stripe
patterns, which can be detected at the fracture or cross section of
the cobalt alloy layer and which elongate perpendicularly to the
film surface, favourably exert an influence on shape anisotropy and
play an important role in the perpendicular anisotropy of the
cobalt alloy layer. According to the results of research carried
out by the present inventors, however, the columnar stripe patterns
are disadvantageous in the light of high internal stress and strain
of the cobalt alloy layer. A disadvantageously large curl of the
conventional perpendicular magnetic recording mediums could be
attributed to the columnar stripe patterns, thus resulting in high
internal stress and strain of such layers.
Conventional perpendicular magnetic recording mediums have been
produced by an RF sputtering method (U.S. Pat. No. 4,210,946).
Namely, cobalt alloy layer containing from 5 to 25 weight % of
chromium is deposited on the base to a thickness of 1 micron by
means of the RF sputtering method. However, the RF sputtering
method, in which the target electrode and the base are disposed
opposite to one another, cannot be applied in the case of
large-scale production of or high-speed growth of perpendicular
magnetic recording layers. The highest growth rate of cobalt alloy
layer achieved at present by means of the RF sputtering method is
about 500 .ANG. per minute (The fourth meeting of the Japan Society
for Applied Magnetism 1980, 60A-4). It is therefore desired that
the growth rate be enhanced to such a degree as to make possible
commercial production of perpendicular magnetic recording mediums.
Furthermore, in the conventional method, the base is heated to
provide the cobalt alloy layer, which grows on the base, with the
columnar stripe patterns mentioned above. When the film is used as
a base for a magnetic recording medium, such as in the case of a
magnetic tape or a floppy disc, the material of which the disc is
made is restricted to a heat-resistant macromolecular material.
Such restriction hinders commercial application of the
perpendicular magnetic recording medium.
It is an object of the present invention to provide a perpendicular
magnetic recording medium which does not have the columnar stripe
pattern but has a novel pattern and a homogenity drastically
reducing the curl of a perpendicular magnetic recording medium and
enhancing the flexibility of such medium.
It is another object of the present invention to provide a method
for producing a perpendicular magnetic recording medium at such an
enhanced rate of production as to make this method commercially
applicable. The method provided should make it possible to use less
expensive and a lower heat-resistant film, such as a polyethylene
terephthalate film and a polyethylene-2,6 naphthalate film, as the
base for the perpendicular magnetic recording medium.
It is yet another object of the present invention to provide a
sputtering device which allows uniform and high-speed formation of
a magnetic layer, particularly a perpendicular magnetic recording
layer.
In accordance with the objects of the present invention, there is
provided a perpendicular magnetic recording medium formed on a base
and comprising an hcp cobalt alloy layer which comprises mainly
cobalt and additionally chromium and which has a direction of easy
magnetization in a direction normal to the base, characterized in
that the cobalt alloy layer has particle pattern, as seen in a
cross section of the layer observed with an electron
microscope.
In accordance with the objects of the present invention, there is
also provided a method for producing a perpendicular magnetic
recording medium, wherein an hcp cobalt alloy layer which consists
mainly of cobalt and additionally chromium and which has a
direction of easy magnetization in a direction normal to the base
is deposited on the base by means of a sputtering method,
characterized in that a magnetic field is generated in a direction
perpendicular to the surfaces of a pair of cathode targets arranged
opposite to one another within a sputtring device, and the cobalt
alloy is deposited on the base which is located beside a space
between said pair of cathode targets and which faces said
space.
In accordance with the objects of the invention, there is also
provided a sputtering device, comprising:
a vacuum vessel;
a pair of oppositely arranged cathode targets in the vacuum
vessel;
a holder for a base, on which a film is deposited by the
sputtering, said holder being located beside a space between the
pair of the cathode targets and facing said space;
a means for generating a magnetic field perpendicularly to the
cathode targets; and,
a power source for applying a negative bias voltage to the
holder.
The embodiments of the present invention are hereinafter explained
with reference to the drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial cross-sectional view of a sputtering device
used to implement the method of the present invention;
FIG. 2 is a drawing illustrating the curl of the perpendicular
magnetic recording medium;
FIG. 3 is a drawing similar to FIG. 1 and schematically illustrates
a sputtering device with magnets mounted behind the cathode
targets;
FIG. 4 is a cross-sectional view of the cathode targets;
FIG. 5 is a cross-sectional view along line V--V' of FIG. 4;
FIG. 6 is a drawing similar to FIG. 3;
FIGS. 7 and 8 are electronmicroscopic photograph of the surface and
cross section of the perpendicular magnetic recording film which
does not have the pattern of the present invention;
FIGS. 9 and 10 are photographs similar to FIGS. 7 and 8,
respectively, but illustrate the film of the present invention;
FIG. 11 shows an example of the columnar pattern;
FIGS. 12 and 13 show examples of the particle pattern of the
present invention;
FIG. 14 is a graph indicating the relationship between saturation
magnetization and alloying contents;
FIG. 15 shows two hysteresis curves of the perpendicular magnetic
recording medium; and,
FIGS. 16 and 17 are graphs illustrating the relationship between
the magnetic properties and the growth rate.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring to FIG. 1, a sputtering device with a pair of opposing
cathode targets is illustrated. As described hereinabove, the
conventional perpendicular magnetic recording mediums were produced
by means of RF sputtering. The sputtering device with a pair of
opposing cathode targets, used to prepare films made of
ferromagnetic non-perpendicular-oriented materials such as iron and
nickel, was recently reported by Naoe et al in the Journal of
Crystal Growth, Vol. 45, pp 361-364, 1978.
The sputtering device with a pair of opposing targets, hereinafter
simply referred to as the opposing target sputtering device,
comprises a vacuum vessel 10 and a pair of cathode targets T.sub.1,
T.sub.2 which are closely attached or secured to the target holders
11, 12. The cathode targets T.sub.1, T.sub.2, hereinafter referred
to as the targets T.sub.1, T.sub.2, are arranged opposite to one
another so that their surfaces, which are subjected to sputtering,
i.e. the sputtering surfaces T.sub.1S, T.sub.2S, face one another
via the space between the targets T.sub.1 and T.sub.2 and are
parallel to one another. The distance between the targets T.sub.1
and T.sub.2 is preferably from 50 to 400 mm. The targets T.sub.1,
T.sub.2, which are subjected to sputtering, are cooled by water,
symbolized by the arrows A and B, which is admitted into the target
holders 11, 12. The target holders 11, 12 are secured to the side
plates 15, 16 of the vacuum vessel via the insulating members 13,
14. The target holders 11, 12 and the insulating members 13, 14 are
protected by shields 17, 18 from plasma particles formed during
sputtering so that abnormal electric discharge does not occur at
the target holders and the insulating members.
The base 20, on which the perpendicular magnetic recording layer is
to be formed by means of the sputtering method, is located on the
base holder 21 disposed beside the targets T.sub.1, T.sub.2 so that
the base 20 is located beside the space between the targets
T.sub.1, T.sub.2 and faces this space. The distance between the
base holder 21 and the ends of the targets T.sub.1, T.sub.2 is
preferably 100 mm or less. The base 20 is usually positioned
vertically.
The field coil 31 is a means for generating the magnetic field
perpendicular to the sputtering surfaces T.sub.1S, T.sub.2S and
surrounds the outer periphery of the vacuum vessel 10. A direct
current from a power source (not shown) is applied to the field
coil 31. A sputtering power source is denoted by 40 and is a direct
current source to which the targets T.sub.1, T.sub.2 and the
shields 17, 18 are connected as a cathode and an anode,
respectively. The vacuum vessel 10 is provided with gas exhausting
port 51 which communicates with a gas exhausting system (not shown)
and a gas introducing port 61 which communicates with a gas source
(not shown) and its associated gas-introduction devices. The arrows
C and D symbolize the flowing direction of the gas.
When operating the opposing target sputtering device described
above, the gas exhausting device is preliminarily operated so as to
satisfactorily withdraw the gas in the vacuum vessel 10 through the
gas exhausting port 51 and, subsequently, a sputtering gas, such as
an argon gas, is admitted into the vacuum vessel 10 so that the
pressure in the vacuum vessel 10 is increased to a predetermined
level, for example from 10.sup.-1 to 10.sup.-4 Torr. Then the field
coil 31 is energized to generate the predetermined magnetic field
H, and the sputtering power source 40 is energized to apply a
predetermined power between the cathode and anode.
In the opposing target sputtering device shown in FIG. 1, the
electric field and the magnetic field H are perpendicular to the
sputtering surfaces T.sub.1S, T.sub.2S. Due to the layout and
configuration of the targets T.sub.1, T.sub.2, the electrons can be
confined in the space between the targets T.sub.1, T.sub.2 while
the metals sputtered from either of the targets T.sub.1, T.sub.2
repeatedly collide with the opposite target and the energy of the
metals is reduced during the repeated collisions. The metals, whose
energy is reduced as just stated, do not seem to deposit on the
base 20 solely due to the energy thereof; rather, the formation of
perpendicular magnetic recording layer seems to result mainly due
to diffusion of the metals from the space between the targets
T.sub.1, T.sub.2 toward the base 20. In other words, a high density
plasma is formed in the space between the targets T.sub.1, T.sub.2
so that diffusion of the metals, which metals are possibly metal
ions, can be realized. It is believed that high-speed growth of the
perpendicular magnetic recording layer can be achieved by
confinement of the electrons or the high density plasma. Since the
base 20 is offset from the targets T.sub.1, T.sub.2, heat
generation due to the impinging effects of the electrons on the
base 20 is not appreciable and therefore perpendicular magnetic
recording film can be formed at a low temperature.
Referring to FIG. 2, the method for determining the curl of the
perpendicular magnetic recording medium is schematically
illustrated. The degree of curl (Kp) is expressed by: ##EQU1##
wherein the symbol l.sub.0 indicates the length of a specimen
without curl and the symbols h.sub.1, h.sub.2 indicate the
deviation of a specimen with curl from both ends of the specimen
without curl. According to their definition in this specification,
the positive polarity of curl is a curl in which the perpendicular
magnetic recording layer (F) and the base are bent outwardly and
inwardly, respectively, while the negative polarity of a curl is a
curl in which the perpendicular magnetic recording layer and the
base are bent inwardly and outward, respectively. According to the
present invention, the absolute degree of curl (Kp) can be not more
than 15%, preferably not more than 10%.
The length of specimen is approximately 10 mm, and the thickness of
base 20 is usual value for magnetic recording, i.e. 100 microns or
less.
The opposing target sputtering device shown in FIGS. 3 is more
advantageous than the device shown in FIG. 1 in regard to the fact
that the magnetic field H is not formed entirely within the vacuum
vessel 10, as it is in the device shown in FIG. 1, but is locally
formed. In other words, in the device shown in FIG. 1, the electron
density is locally high around the central axis across the targets
T.sub.1, T.sub.2. Contrary to this, the electron density is uniform
in the space between the targets T.sub.1, T.sub.2 in the device
shown in FIG. 3. In FIG. 3, the same reference numerals as in FIG.
1 denote the same members. Reference 22 denotes an insulating
member which electrically insulates the base 20 from the vacuum
vessel 10, and the base a bias power source 41 (FIG. 4). A shutter
(not shown) disposed within the vacuum vessel 10 advances into the
gap between the base 20 and the targets T.sub.1, T.sub.2 and
protects the base 20 from the pre-sputtering gases. The shutter
retracts from the gap upon completion of pre-sputtering.
In the opposing target sputtering device shown in FIG. 3 the
perpendicular magnetic field is generated only between the targets
T.sub.1, T.sub.2. The generation of such perpendicular magnetic
field is made possible by locating permanent magnets 32, 33 behind
the targets T.sub.1, T.sub.2. The permanent magnets 32, 33 (FIGS. 4
and 5) are located in such a manner that the N pole of the
permanent magnet 32 situated behind the target T.sub.1 is opposite
to the S pole of the permanent magnet 33 situated behind the target
T.sub.2. The magnetic field between the N and S poles mentioned
above is, therefore, perpendicular to the targets T.sub.1, T.sub.2
and is also confined between the targets T.sub.1, T.sub.2. The
targets T.sub.1, T.sub.2 and the permanent magnets 32, 33 are
cooled by a cooling medium, such as water, admitted into the inner
space of the target holders 11, 12 via the cooling medium conduits
151, 152. The magnetic field formed solely between the targets
T.sub.1, T.sub.2 results in the uniform distribution of electrons
in the space between the targets T.sub.1, T.sub.2. Since the
density of plasma particles in such space is uniformly high,
sputtering of the targets T.sub.1, T.sub.2 is accelerated and the
diffusion of metals from such space is enhanced, with the result
that the deposition rate of the metals on the base 20 is further
increased as compared with the opposing target sputtering device
shown in FIG. 1. The arrangement of the permanent magnets 32, 33
behind the targets T.sub.1, T.sub.2 is advantageous from an
industrial point of view because the structure of the opposing
target sputtering device is simplified. In addition, the
arrangement of the permanent magnets 32, 33 solely along the
periphery of the targets T.sub.1, T.sub.2 results in a local
distribution of the magnetic field along such periphery. The
results of the experiments of the present inventors revealed that
the entire surface of the targets T.sub.1, T.sub.2 can be virtually
uniformly subjected to sputtering due to this local distribution of
the magnetic field. This is very advantageous because sputtering
efficiency can be enhanced or high-speed sputtering can be achieved
without causing a local elevation of temperature in the targets
T.sub.1, T.sub.2. Furthermore, the permanent magnets can be located
entirely behind the targets T.sub.1, T.sub.2.
As can be understood from the permanent magnets 32, 33 shown in
FIGS. 4 and 5, said magnets are hollow and have a rectangular cross
section. It can be understood from FIGS. 4 and 5 that the magnetic
field between the targets T.sub.1, T.sub.2 completely surrounds the
outer periphery of the space between the targets T.sub.1,
T.sub.2.
In the opposing target sputtering devices shown in FIGS. 3 and 4, a
bias potential is applied to the base 20 from the bias power source
41, which is an alternating current source in FIG. 4 but may be a
direct current source. The bias potential of the base 20, which is
lower than the ground potential, generates an electric field which
is virtually perpendicular to the base 20. The frequency of
alternating current should be RF frequency. It is supposed that the
rate of deposition of metals on the base 20 and the
crystallographic properties of the cobalt alloy can be improved by
the bias potential.
In the opposing target sputtering device shown in FIG. 6, the base
20 is in the form of a film and the base holders 21 are in the form
of a roll. In FIG. 6, the same reference numerals as in FIG. 1
denote the same members. The reference numerals 44 and 45 denote an
evacuating system and a gas-introducing system, respectively. The
unwinding roll 21a, the supporting roll 21b and the winding roll
21c are rotatably mounted on each bracket (not shown) and are
aligned so that their axes are parallel to each other. The surface
of the base 20 to be subjected to the deposition of metals thereon
is successively conveyed to face the space between the targets
T.sub.1, T.sub.2 by the unwinding and winding rolls 21a and 21c,
respectively, while the supporting roll 21b supports said surface
virtually perpendicularly to the surface of the targets T.sub.1,
T.sub.2.
When using opposing target sputtering devices such as those shown
in FIGS. 1, 3, and 6, a less expensive and lower heat-resistant
macromolecular material than that used in the conventional method
can be used for the base 20. Such macromolecular material includes
polyester, such as polyethylene terephthalate and polyethylene-2,6
naphthalate, and other organic macromolecular materials having heat
resistance at approximately 100.degree. C. at the highest. These
materials are usually used as a flexible base in the film form. The
polyethylene terephthalate and polyethylene-2,6 naphthalate
mentioned above include not only homopolymers but also copolymers,
in which 85% or more of the repeating units are polyethylene
terephthalate or polyethylene naphthalate.
The theory of forming the perpendicular magnetic recording medium
by means of the opposing target sputtering device (FIGS. 1, 3, and
6) is now explained although the scope of the present invention is
not limited to such theory.
The sputtering gas, usually argon gas ionized as Ar.sup.+ ions,
which is accelerated due to the cathode drop in front of the
targets T.sub.1, T.sub.2, impinges the sputtering surfaces
T.sub.1s, T.sub.2s, with the result that gamma electrons are
expelled from the sputtering surfaces T.sub.1s, T.sub.2s. The
magnetic field is generated between the targets T.sub.1, T.sub.2
perpendicular to the sputtering surfaces T.sub.1s, T.sub.2s, and
the electric field at the cathode drop space in front of the
targets is directed parallel to the magnetic field. The direction
of the electric field is the same as and opposite to the magnetic
field. The gamma electrons expelled from either of the targets
T.sub.1, T.sub.2 are moved toward the opposite target while the
gamma electrons are confined in the space between the targets
T.sub.1, T.sub.2. The gamma electrons are then reflected from the
opposite target while simultaneously being confined in said space.
During the reciprocal movement of the gamma electrons, the gamma
electrons collide with the neutral gases so that these gases are
ionized and electrons are simultaneously expelled from the gases.
The so-ionized gases promote the emission of gamma electrons from
the targets T.sub.1, T.sub.2 and, in turn, ionization and electron
formation occur. A high density plasma is, therefore, formed in the
space between the targets T.sub.1, T.sub.2, resulting in an
increase in sputtering of the cobalt alloy material from the
targets T.sub.1, T.sub.2 and an increase in the deposition rate. As
is described in reference to the illustration of FIG. 1, the
opposing target sputtering device has been used to form
ferromagnetic material films for magnetic heads. However, these
films do not have perpendicular anisotropy. The present inventors
discovered that a perpendicular magnetic medium can be produced by
means of opposing target sputtering devices. And the production
rate is considerably higher than that by means of conventional
sputtering devices. The fact that the magnetic layer has
perpendicular anisotropy appears to be due to the confinement of
the gamma electrons and the position of the base, that is, the base
is positioned next to the space between the targets T.sub.1,
T.sub.2 where the influence of the argon ions, the gamma electrons
and the secondary electrons is not very appreciable. In other
words, the base 20 is not exposed to the plasma gas between the
targets T.sub.1, T.sub.2. However, the metals of the cobalt alloy,
which are sputterd from the targets T.sub.1, T.sub.2 can be
deposited on the base 20 at a high rate. This appears to be
realized by the fact that sputtering is promoted as stated above.
The diffusion of metals from the space between the targets T.sub.1,
T.sub.2 toward the base 20 takes place, and the metals are cooled
during diffusion so that the metal particles formed during the
cooling process are very liable to form an hcp structure and a
homogeneous layer on the base 20. Since the kinetic energy of the
diffused metals is considerably less than that of the metals
directly after sputtering, the kinetic energy of metals deposited
on the base is very low, which seems to result in perpendicular
anisotropy and low degree of curl (Kp).
In the method of the present invention, the base is advantageously
subjected to bombardment in a glow discharge before sputtering.
In the opposing target sputtering device shown in FIG. 1, an
electromagnetic force (f) is generated between the magnetic field
(H) and the current (i) which is generated by motion of the
secondary electrons, gamma electrons, argon ions and the ionized
metal particles. The electromagnetic force (f) is expressed by:
The argon ions are subjected to the electromagnetic force (f) when
they diffuse from the space between the targets T.sub.1, T.sub.2
toward the base 20. However, the electromagnetic force (f) is
decreased as the argon ions are diffused. The direction of the
electromagnetic force (f) is determined by Fleming's left-hand
rule, and it is inclined relative to the surface of the base 20.
The argon ions therefore impinge on the base 20 from the inclined
direction, which seems to be undesirable for the uniform formation
of perpendicular magnetic recording layer. Contraty to this, in the
opposing target sputtering devices shown in FIGS. 3 and 4 and
provided with a bias power source 41, the bias potential is applied
to the base 20 and its surface is kept electrically neutral during
the sputtering so that the ionized metal particles can impinge on
the base 20 in a direction which is virtually perpendicular to the
base 20. The impinging kinetic energy is proportional to the bias
potential, which generates the electric field perpendicular to the
base 20. Perpendicular impinging of the ionized metal particles
under an appropriate kinetic energy, especially at the beginning of
sputtering, is advantageous for perpendicular anisotropy of cobalt
alloy and also for a small degree of curl. Such kinetic energy can
be ensured by a bias potential of not more than -100 V.
The present inventors also studied the pattern of perpendicular
magnetic layers and its influence upon the properties of the
layers, particularly the curl and flexibility of magnetic recording
mediums.
The pattern of magnetic recording layers was investigated by means
of a diffraction electronmicroscope produced by Japan Electron Co.,
Ltd (JSM-35C type). The crystallographic structure of the magnetic
recording mediums was identified by means of a scaling X-ray
diffractometer produced by Rigakudenki Co., Ltd. The degree of
perpendicular orientation of the magnetic layers was determined by
subjecting these layers to X-ray diffraction, obtaining the rocking
curve of the (002) planes of the hcp structure, and measuring the
half value width .DELTA..theta..sub.50. The perpendicular coercive
force Hcv and anisotropy field Hk of the perpendicular magnetic
layers were measured by means of a vibration sample magnetometer
(VSM) produced by Toeikogyo K.K. The method of measuring the
anisotropy field Hk was based on the method reported in IEEE
TRANSACTIONS OF MAGNETICS, VOL. MAG-16, No. 5, SEPT. 1980, page
1,113. FIG. 15 shows the method of illustrating the anisotropy
field Hk. The solid lines and the broken lines indicate the
perpendicular hysteresis and the in-plane hysteresis, respectively.
An effective anisotropy field Hkeff is obtained by drawing the
maximum permeability line of the in-plane hysteresis and then
determining the intersecting line of the maximum permeability line
with the perpendicular hysteresis curve.
Referring to FIGS. 7 and 8, the pattern of a perpendicular magnetic
recording layer produced by a conventional sputtering device is
shown. This layer was produced under the following conditions:
A. The Sputtering Device
A DC magnetron sputtering device was equipped with a base holder
and target arranged oppositely to one another in the vacuum vessel.
A bias potential of -100 V was applied to the base holder.
B. Base:
A 75-micron thick polyimide film.
C. Thickness of the Magnetic Film:
0.8 micron
Referring to FIGS. 9 and 10, which are electron microscopic
photographs similar to FIGS. 7 and 8, respectively, an example of
the pattern of a perpendicular magnetic recording layer according
to the present invention is shown. This layer was produced under
the following conditions:
A. The Opposing Target Sputtering Device (FIG. 3)
(1) Material of the Targets T.sub.1, T.sub.2 : cobalt alloy
containing 17% by weight (18.5 atomic %) of chromium
(2) Distance Between the Targets T.sub.1, T.sub.2 : 100 mm
(3) Magnetic Field in the Neighborhood of the Targets T.sub.1,
T.sub.2 : 150.about.300 gauss
(4) Dimension of the Targets T.sub.1, T.sub.2 : 150 mm.times.100
mm.times.5 mm (thickness)
(5) Distance of the Base 20 from the End of the Targets T.sub.1,
T.sub.2 : 35 mm
B. Base
a 75-micron thick polyimide film
C. Thickness of the Magnetic Layer:
1.3 microns
The perpendicular magnetic recording layer was produced by the
following procedure.
The base 20 was first fixed on the base holder 21 and then the gas
in the vacuum vessel was evacuated until an ultimate degree of
vacuum of 1.times.10.sup.-6 Torr or less could be achieved.
Subsequently, the argon gas was admitted into the vacuum vessel 10
until the pressure was increased to 4 mm Torr. After pre-sputtering
amounting to 3 to 5 minutes, the shutter (not shown in FIG. 3) was
retracted and the formation of a perpendicular magnetic recording
layer on the base was initiated.
Several properties of the magnetic mediums so produced are given in
Table 1.
TABLE 1
__________________________________________________________________________
Degree Magnetic Properties Crystals of Curl Hkeff Samples Structure
.DELTA..theta..sub.50 (.degree.) Kp (%) Hcv(Oe) (KOe) Ms(emu/cc)
__________________________________________________________________________
Invention hcp (002) 3.0 +3 930 5.4 550 (FIGS. 9 and 10) Comparative
Example hcp (002) 5.6 +17 1100 5.4 565 (FIGS. 7 and 8)
__________________________________________________________________________
The magnetic properties of both samples very well satisfy the
requirements for the magnetic properties in the perpendicular
magnetic recording layer mentioned hereinabove in the description
of the background of the invention. However, the degree of curl
(Kp) of the present invention is +3%, a very small degree, while
the degree of curl (Kp) of the comparative example is +17%, a very
large degree. If the magnetic medium of the comparative example is
commercially applied, the spacing loss between the magnetic head
and the magnetic medium is so large that the electromagnetic
conversion characteristic is disadvantageously decreased.
The deposition rate of cobalt alloy in the method of the invention
using the opposing target sputtering device was 0.10 micron (1000
.ANG.) per minute, while the deposition rate in the comparative
example using the conventional sputtering device was 0.02 micron
(200 .ANG.) per minute. When the method of the present invention is
compared with the method of the comparative example, it can be
concluded that the deposition rate of the present invention is five
times as high as that of the comparative example and achieves a
degree of curl (Kp) superior to and magnetic properties equivalent
to those of the comparative example.
When FIGS. 7 and 8 of the comparative example and FIGS. 9 and 10 of
the present invention are compared, the following conclusion can be
obtained.
The crystals of both samples are hcp and the half value widths
(.DELTA..theta..sub.50) of both samples are not largely different
from one another. In addition, the surface pattern of both samples
shown in FIGS. 7 and 8 shows fine particles which aggregate next to
each other forming grain boundaries. The surface pattern of the
perpendicular magnetic recording layer according to the present
invention is not different from that of the comparative example.
The diameter of the fine particles shown in FIG. 9 is almost
uniform and is approximately 500 .ANG. on the average. Contrary to
the similarity between the surface patterns, of the present
invnetion and the comparative example the cross section pattern
shown in FIG. 10 (the present invention) is completely different
from that shown in FIG. 9 (the comparative example).
In FIG. 8, the stripe patterns are elongated longitudinally and
perpendicularly to the layer surface, and the border between
neighbouring longitudinal patterns is similar to a crack. In FIG.
10, the longitudinal elongated patterns cannot be detected;
instead, the cross section is composed of particles, each particle
having a similar dimension in any direction. These particles have
an irregular shape, e.g. polygonal, ellipsoidal and the like, but
definitely do not have a columnar shape. The dimension of all of
the particles is not constant throughout the cross section; it
tends to be larger in the neighborhood of the layer surface than in
the neighborhood of the base. The dimension of the particles in
terms of a circumscribed circle of the particles is approximately
2000 .ANG. at the maximum. Particles with such a dimension are
present near the layer surface. Also, judging from the comparison
of the pattern shown in FIG. 10 with the columnar pattern
previously reported in several technical reports and papers, the
pattern shown in FIG. 10 is distinctly different from the columnar
pattern.
It has been previously believed in the art of perpendicular
magnetic recording mediums that perpendicular anisotropy is mainly
attributable to a columnar pattern. Surprisingly, however,
perpendicular magnetic recording film, which does not have a
columnar pattern but a particle pattern, possesses magnetic
properties equivalent to those of conventional perpendicular
magnetic recording layers having a columnar pattern and even
possess a degree of curl which is less than that of conventional
layers. These merits seem to be possible for the following reasons.
Non-columnar particles also have an axis of easy magnetization (the
C axis of hcp cobalt) normal to the layer surface, and the internal
stress or strain induced during their deposition is extremely
reduced in comparison with the internal stress or strain induced
during the deposition of cobalt alloy having a columnar structure.
In the non-columnar pattern discovered by the present inventors, it
should not be construed that colummnar crystals cannot be formed at
all. Rather, it is reasonable to construe that the boundary layer
of columnar crystals is too thin to be detected by an electron
microscope. Since the boundary layers, which resemble cracks, in
the conventional perpendicular magnetic layers cannot be detected
in the layers of the present invention, one can conclude that the
cobalt alloy crystals are very homogeneous in the present
invention. Furthermore, since the information to be written in the
perpendicular magnetic layer can be recorded in the cobalt alloy
crystal grains but cannot be recorded in the boundary layers, a
high recording density could be achieved in the perpendicular
recording mediums of the present invention. Also, the low
flexibility of the conventional perpendicular magnetic recording
mediums may be ascribed to the boundaries, which, however, cannot
be detected in such mediums of the present invention.
The non-columnar pattern is hereinafter referred to as particle
pattern, in which the particles exhibit similar or not greatly
different dimensions in all directions. However, it is to be noted
that "particle pattern" indicates a similarity of dimensions as
seen solely in the cross section of a layer. Such pattern is
similar to the pattern of equiaxed particles.
Other examwples of columnar pattern and particle pattern are
further explained. Referring to FIG. 11, an example of a columnar
pattern is shown. In this pattern, the degree of curl is inferior
to that of the particle pattern. The particle pattern shown in
FIGS. 12 and 13 exhibits an excellent anti-curling tendency. In
particle pattern, the particle size is not uniform throughout the
cross section of the perpendicular magnetic recording layer, but
the particles exhibit similar dimensions in all directions as seen
in the cross section.
The specimens for observing the surface structure (FIGS. 8 and 10)
were prepared by depositing an Au-Pb layer on the perpendicular
magnetic recording layers to a thickness of approximately 200
.ANG.. Electronmicroscopic photographs were taken at a
magnification of 40,000 and under an acceleration voltage of 25 kV.
The specimens for observing the cross sectional pattern (FIGS. 8
and 10) were prepared by putting the magnetic recording mediums
into a gelation capsule together with ethyl alcohl, cooling the
capsule with liquid nitrogen for two hours, and then cleaving the
capsule with a cleaving knife. The device used for the
freeze-cleaving method was a TF-1 type device produced by Eiko
Engineering Co. Ltd.
The cobalt alloy used in the present invention as the material for
a perpendicular magnetic recording layer is mainly composed of
cobalt and additionally chromium. Another additional element or
elements, which do not alter the hcp structure, may be incorporated
into the cobalt alloy. The present inventors discovered that
rhenium, tungsten and molybdenum which can be incorporated into the
cobalt alloy in addition to chromium in amounts of from 2 to 10
atomic %, cannot alter the hcp structure of cobalt; rather, these
elements advantageously decrease the half value width
(.DELTA..theta..sub.50) and increase the deposition rate of cobalt
alloy. The atomic percentage of chromium(x) and the atomic
percentage of rhenium, tungsten and molybdenum(y) should have the
following values:
Although three alloying elements are described, other elements
which do not alter the hcp structure by forming a second phase
might be contained in the cobalt alloy.
Referring to FIG. 14, the solid line indicates the saturation
magnetization of Co-Cr alloys and the spots of symbols "O" indicate
the saturation magnetization of Co-Cr-Re alloys with the total
contents of chromium and rhemium given in the drawiwng. When
perpendicular magnetic layer of these alloys is formed by the DC
magnetron sputtering, the degree of curl is large, while the degree
of curl can be decreased by the present invention.
The perpendicular magnetic recording medium according to the
present invention may comprise, in addition to the base and the
perpendicular magnetic recording layer (cobalt alloy film), a layer
of soft magnetic metal. The layer of soft magnetic metal may be
formed on the surface of the base opposite to the surface where the
perpendicular magnetic recording film is formed. Alternatively, the
layer of soft magnetic metal may be formed beneath the
perpendicular magnetic recording film. The soft magnetic metal
herein indicates crystalline ferromagnetic metal having a coercive
force of 50 Oe or less or, preferably, 10 Oe or less and high
permeability, such as be Permalloy, Alperm and Sendust. The
thickness of soft magnetic metal layer should be from 0.10 to 1
micron. A layer of soft magnetic metal can furthermore decrease the
degree of curl (Kp).
The base of the perpendicular magnetic recording medium may be made
of metal, glass, plastics or other materials having a heat
resistance sufficient for withstanding the sputtering.
Particularly, organic macromolecular film, such as polyester film,
having a lower heat resistance than that of polyimide or polyamide
film can be used in the present invention. The organic
macromolecular film may contain an inactive inorganic compound,
such as MgO, ZnO, MgCO.sub.3, CaCO.sub.3, CaSO.sub.4, BaSO.sub.4,
Al.sub.2 O.sub.3, SiO.sub.2, or TiO.sub.2, for the purpose of
adjusting the surface roughness of the film.
In addition, a lubricant may be applied to the surface of the base
opposite to the surface where the perpendicular magnetic recording
film is formed. The lubricant may be an organic lubricant, e.g.
sorbitan, an organic macromolecular lubricant, e.g.
polytetrafluoroethylene or polyethylene, or an inorganic lubricant,
e.g. alumina, kaolin, silica or molybdenum sulfide. The application
of a lubricant is advisable when the film bases do not slide
favorably in relation to one another.
The properties of the perpendicular magnetic recording film adapted
for use in combination with the current magnetic heads, such as a
single pole type head and a ring type head include: a half value
width .DELTA..theta..sub.50 .ltoreq.8.degree., a perpendicular
coercive force Hcv.gtoreq.500 Oe, a ratio of Hcv/Hch.gtoreq.2.0,
and an anisotropy field Hk.gtoreq.4KOe. These properties can be
readily achieved in the present invention, as will be understood
from the description of the Examples.
The present invention is now explained by way of Examples.
EXAMPLE 1
Samples of the perpendicular magnetic recording medium were
prepared under the following conditions.
A. The Opposing Target Sputtering Device (FIG. 3)
(1) Material of the Targets T.sub.1, T.sub.2 : cobalt alloy
containing 17% by weight (18.5 atomic %) of chromium
(2) Distance Between the Targets T.sub.1, T.sub.2 : 100 mm
(3) Magnetic Field in the Neighborhood of the Targets T.sub.1,
T.sub.2 : 150.about.300 gauss
(4) Dimension of the Targets T.sub.1, T.sub.2 : 150 mm.times.100
mm.times.5 mm (thickness)
(5) Distance of the Base 20 From the Ends of the Targets T.sub.1,
T.sub.2 : 35 mm
B. Base 20:
a 75 mm thick polyester film
C. Thickness of Cobalt Alloy Layer:
1.3 microns
The perpendicular magnetic recording layer was produced by the
followirng procedure.
The base 20 was first fixed on the base holder 21 and then the gas
in the vacuum vessel was evacuated until an ultimate degree of
vacuum of 1.times.10.sup.-6 Torr or less could be achieved.
Subsequently, an argon gas was admitted into the vacuum vessel 10
until the pressure was increased to 4 mm Torr. After pre-sputtering
for 3 to 5 minutes, the shutter (not shown in FIG. 3) was retracted
and the formation of a perpendicular magnetic recording layer on
the base was initiated.
Several properties of the magnetic mediums so produced are given in
Table 1.
The properties of the prepared samples were measured as stated
above in the description of the perpendicular magnetic recording
films with the columnar or particle-pattern.
The properties of the samples in this Example are given in the
following table.
TABLE 2 ______________________________________ Thickness Crystal
Degree Perpendicular Sample of Base Orien- of curl Coercive Force
Nos. (.mu.m) tation .DELTA..theta..sub.50 Kp (%) Hcv (Oe)
______________________________________ 1 16 (002) 5.0.degree. 7
1050 2 14 (002) 4.3.degree. 8 1030 3 12 (002) 4.5.degree. 10 1000 4
10 (002) 3.2.degree. 14 1100
______________________________________
The samples (the cleaved surface of the samples) were of particle
pattern. The degree of curl (Kp) was increased in accordance with
the decrease in the thickness of the base, which was polyester
film. However, in Sample 4, the degree of curl (Kp), which was 14%
was not unfavorable, because the perpendicular magnetic recording
layer (cobalt alloy layer) had flexibility as will be explained
later.
EXAMPLE 2
Samples of the perpendicular magnetic recording medium were
prepared by successively forming on a base of Permalloy magnetic
layer and a cobalt alloy layer under the following conditions.
A. Formation of Permalloy Layer
(1) Opposing Target Sputtering: device used in Example 1
(2) Targets: permalloy plates of 80 wt% of nickel and 20 wt% of
iron
(3) Operating Condition: an argon pressure of 1.times.10.sup.-2
Torr and a deposition rate of 400 .ANG./minute
(4) Obtained Permalloy Layer: the layer had a coercive force of 16
Oe
and a thickness of 0.44 microns (.mu.m).
B. Formation of Co alloy Layer
The Co alloy Layer was formed under the same conditions as in
Sample 3 of Example 1.
The electromagnetic conversion characteristic of the sample
prepared in the present Example (Sample 5) was evaluated by means
of the magnetic head of the perpendicular magnetic recording mode,
in which the main and auxiliary electrodes are opposed to one
another.
TABLE 3 ______________________________________ Recording 1 10 50
100 150 Density (KBPI) Output (S/N) (dB) 35 35 25 16 8
______________________________________
The electromagnetic conversion characteristic in terms of the
output of magnetic head did not vary appreciably after the
perpendicular magnetic recording medium (Sample 5) had been used
repeatedly a thousand times, thereby proving the favarable
flexibility of the magnetic tape. The cross-sectional pattern of
Sample 5 was particle.
EXAMPLE 3
Samples of the perpendicular magnetic recording medium were
prepared under the following conditions.
A. Opposing Target Sputtering Device (FIG. 6)
(1) Material of the Targets T.sub.1, T.sub.2 : cobalt alloy
containing 17% by weight (18.5 atomic %) of chromium.
(2) Base 20: a 75-micron thick polyimide film
(3) Distance Between the Targets T.sub.1 and T.sub.2 : 100 mm
(4) Magnetic Field in the Neighborhood of the Targets T.sub.1,
T.sub.2 : 150.about.300 gauss
(5) Dimension of the Targets T.sub.1, T.sub.2 : 300 mm.times.125
mm.times.5 mm (thickness)
(6) Distance of the Base from the Ends of the Targets T.sub.1,
T.sub.2 : 30 mm
B. Operation Procedure
The cobalt alloy layer was formed by successively performing the
following procedures.
(1) The base 20 was fixed on the base holder 21 and then the vacuum
vessel 10 was evacuated until an ultimate degree of vacuum of
2.times.10.sup.-6 Torr or less was achieved.
(2) Argon gas was admitted into the vacuum vessel 10 until the
pressure of 4 mm Torr or 1.5 mm Torr was obtained. After the
pre-sputtering for 3 to 5 minutes, the shutter (not shown in FIG.
6) was retracted, thereby exposing the base 20 to the plasma gas.
The base was held in a stationary position.
Several properties of the samples are given in Table 4.
TABLE 4
__________________________________________________________________________
Preparation Degree Conditions Half Magnetic of Deposition Ar Gas
Value Properties Curl Sample Rate Pressure Width Hcv Hkeff Kp No.
(.ANG./min) (mm Torr) .DELTA..theta..sub.50 (Oe) Hcv/Hch (KOe) (%)
__________________________________________________________________________
6 4290 4 6.5.degree. 1220 2.7 4.5 +11 7 2300 4 6.2.degree. 1100 2.5
4.8 +9 8 1150 4 5.0.degree. 750 2.6 5.3 +5 9 1610 1.5 5.0.degree.
1020 2.8 5.0 +8
__________________________________________________________________________
The symbol of "Hch" in Table 4 indicates the horizontal coercive
force.
As is apparent from Table 4, the excellent perpendicular magnetic
layers can be prepared by means of a method which achieves high
deposition rate. In addition, the degree of curl (Kp) is
advantageously low.
EXAMPLE 4
The same procedure as in Example 3 was repeated; however, the base
was a 50 micron polyester film and the argon gas pressure was that
given in Table 5.
Several properties of the samples are given in Table 5.
TABLE 5
__________________________________________________________________________
Preparation Conditions Half Magnetic Degree of Deposition Argon Gas
Value Properties Curl Sample Rate Pressure Width Hcv Hkeff Kp Nos.
(.ANG./min) (mm Torr) .DELTA..theta..sub.50 (Oe) Hcv/Hch (KOe) (%)
__________________________________________________________________________
10 1090 8 6.4.degree. 660 2.6 4.8 +5 11 1200 4 5.0.degree. 600 2.6
4.7 +5 12 1170 1.5 4.5.degree. 550 2.7 4.5 +6
__________________________________________________________________________
As is apparent from Table 5, the deposition rate is high, the
degree of curl (Kp) is low and a polyester film having a low heat
resistance can be used as the base of a perpendicular magnetic
recording medium. In addition, the argon gas pressure can be varied
in a broad range.
EXAMPLE 5
The same procedure as in Example 3 was repeated. However, in the
operation procedure (2) an ion bombardment was carried out prior to
depositing the cobalt alloy on the base (75 micron thick polyimide
film). For the ion bombardment, the argon gas pressure was adjusted
to 50 mm Torr and an alternating voltage of 300 V was applied
between the shutter and the base for a period of three minutes,
thereby inducing a glow discharge and ion bombardment of the base.
Some of the bases of the samples in the present examples were
subjected to degassing at 280.degree. C. for a period of 60 minutes
in vacuum.
Several properties of the samples are given in Table 6.
TABLE 6
__________________________________________________________________________
Preparation Degree Condition Half Magnetic of Deposition Argon Gas
Value Properties Curl Sample Rate Pressure Width Hc Hcv/ Hkeff Kp
Nos. (.ANG./min) (mm Torr) .DELTA..theta..sub.50 (Oe) Hch (KOe) (%)
Degassing
__________________________________________________________________________
13 1190 1.5 2.9.degree. 700 2.6 5.4 +11 x 14 1280 1.5 2.9.degree.
940 2.6 5.4 +6 o 15 1260 4 3.7.degree. 1060 3.1 5.7 +6 o 16 2000 4
3.4.degree. 850 2.4 5.4 +10 x 17 2410 4 2.8.degree. 1120 2.4 5.9
+11 o 18 520 4 4.6.degree. 820 2.5 5.5 +4 x 19 590 4 4.7.degree.
990 2.8 5.7 +8 o 20 3690 4 3.1.degree. 1150 3.3 5.5 +11 x 21 3910 4
3.0.degree. 1150 2.7 5.6 +11 o
__________________________________________________________________________
Such gaseous components as moisture, water and the like, were
analyzed with a mass spectrograph (SM-800 type produced by Japan
Vacuum Enginnering), when an ultimate degree of vacuum was attained
during the formation of the perpendicular magnetic recording
layers. The gas pressures of moisture (H.sub.2 O) and oxygen
(O.sub.2) were 1.5.times.10.sup.-6 Torr and 8.times.10.sup.-8 Torr,
respectively. The degree of vacuum in the present example was,
therefore, about one tenth as low as about 2.times.10.sup.-7 Torr
which is the degree of vacuum required for the conventional RF
sputtering. This degree of vacuum would be advantageous from a
commercial point of view, because the perpendicular magnetic
recording layers could be produced economically by a vacuum
evacuation system with low capacity. The properties of the
perpendicular magnetic recording layers were not significantly
influenced by the degassing. Degassing, which is allegedly
indispensable for polyimide films in conventional RF sputtering,
can therefore be omitted according to the present invention.
EXAMPLE 6
The procedure of Example 1 was repeated. However, the argon gas
pressure was 4 mm Torr and the deposition rate varied. The results
are given in FIG. 16, wherein the abscissa indicates the deposition
rate (R) and the ordinate indicates the perpendicular coercive
force (Hcv), the antisotropy field (Hkeff) and the half value width
(.DELTA..theta..sub.50). As is apparent from FIG. 16, an increase
in the deposition rate (R) of up to 4000 .ANG. per minute does not
result in deterioration of the magnetic properties required for the
perpendicular magnetic recording mediums; but rather the
perpendicular coercive force Hcv is increased at a higher
deposition rate.
The temperature of the base was normal temperature (about
20.degree. C.) at the beginning of sputtering and was not
intentionally elevated. Without intentional heating of the base,
which has been believed to be indispensable in the RF sputtering,
the cobalt alloy layers having good perpendicular magnetic
recording properteis could be produced at a high deposition rate of
4000 .ANG. per minute.
EXAMPLE 7
The same procedure as in Example 5 was repeated. However, the argon
gas pressure was 4 mm Torr and the deposition rate of cobalt alloy
layer was varied up to a level of approximately 4000 .ANG./min. The
results are given in FIG. 17. The tendency of the magnetic
properties and half value width depending on the deposition rate
(R) shown in FIG. 17 is similar to that shown in FIG. 16. However,
in FIG. 17, the half value width (.DELTA..theta..sub.50) is
advantageously decreased with an increase in the deposition rate
(R), which is due to the bombardment in a glow discharge in the
present Example.
The degree of curl (Kp) shown in FIG. 17 is acceptable, although it
increases high at a high deposition rate (R) of about 4000
.ANG./min.
EXAMPLE 8
Samples of the perpendicular magnetic recording medium were
prepared under the following conditions.
A. Opposing Target Sputtering Device (FIG. 4)
(1) Material of the Targets T.sub.1, T.sub.2 : cobalt alloy
containing 17% by weight of chromium
(2) Base 20: a 75 micron thick polyimide film
(3) Distance Between the Targets T.sub.1 and T.sub.2 : 100 mm
(4) Magnetic Field in the Neighborhood of Targets T.sub.1, T.sub.2
: 100.about.150 gauss
(5) Dimension of the Targets T.sub.1, T.sub.2 : 100 mm in
diameter.times.5 mm in thickness
(6) Distance of Base 20 from the Ends of the Targets T.sub.1,
T.sub.2 : 25 mm
(7) Bias Power Source 41: RF current of 13.56 MHz
B. Operation Procedure
(1) The base 20 was fixed on the base holder 21 and then the vacuum
vessel 10 was evacuated until an ultimate degree of
2.times.10.sup.-6 Torr or less was achieved.
(2) Argon gas was admitted into the vacuum vessel 10 until a
pressure of 4 mm Torr was obtained. Sputtering was carried out at
an argon gas pressure of 4 mm Torr while power of 500 W was applied
between the targets T.sub.1, T.sub.2 and shields 17, 18. A 1-micron
thick cobalt alloy layer was formed on each base 20. Several
properties of the samples are given in the following table.
TABLE 7
__________________________________________________________________________
Preparation Conditions Voltage of Half Degree of Bias Power
Deposition Value Magnetic Properties Curl Sample Source Rate Width
Hcv Hkeff Kp Nos. (V) (.ANG./min) .DELTA..theta..sub.50 (Oe)
Hcv/Hch (KOe) (%)
__________________________________________________________________________
22 0 1150 5.0.degree. 750 2.6 5.3 +5.0 23 -25 1150 2.8.degree. 970
3.8 5.6 +5.2 24 -50 1130 2.7.degree. 1030 4.0 5.3 +6.0 25 -75 1090
2.7.degree. 950 4.3 5.1 +8.0 26 -100 830 12.4.degree. 890 2.6 4.1
+13.0
__________________________________________________________________________
In Table 7, the bias voltage was applied to all the samples except
for Sample 22. The half value width (.DELTA..theta..sub.50), which
indicates the degree of C axis orientation of the cobalt alloy, can
be improved by a bias voltage having an absolute value of less than
100 V. The application of a bias voltage of -100 V results in a
decrease in the deposition rate, deterioration of the degree of C
axis orientation, and an increase in the degree of curl (Kp).
EXAMPLE 9
The procedure of Example 8 was repeated. However, the bias voltage
from the bias power source 41 was applied only during the growth
period of the cobalt alloy layer up to a thickness of 0.1 .mu.m,
and during the remaining growth period, namely the growth of the
cobalt alloy layer from 0.1 to 1.0 .mu.m, the bias voltage was kept
at 0 V. The results are given in the following table.
TABLE 8
__________________________________________________________________________
Preparation Conditions Voltage of Half Degree of Bias Power
Deposition Value Magnetic Properties Curl Sample Source Rate Width
Hcv HKeff Kp Nos. (V) (.ANG./min) .DELTA..theta..sub.50 (Oe)
Hcv/Hch (KOe) (%)
__________________________________________________________________________
27 -75 1100 2.5.degree. 950 3.5 5.5 5.0 28 -100 1050 2.7.degree.
1000 3.8 5.5 5.3
__________________________________________________________________________
The obtainted half value width (.DELTA..theta..sub.50) and degree
of curl (Kp) are very desirable as the properties of the
perpendicular magnetic recording mediums.
EXAMPLE 10
The procedure of Example 8 was repeated. However, the Permalloy
layer was formed before the formation of the cobalt alloy layer
under the following conditions.
A. Opposing Target Sputtering Device (FIG. 3)
(1) Material of the Targets T.sub.1, T.sub.2 : Ni-Fe Permalloy (22%
by weight of iron)
(2) Base 20: 25 micron thick polyester film
(3) Magnetic Field in the Neighborhood of the Targets T.sub.1,
T.sub.2 : 250.about.300 gauss
(4) Distance of Base 20 from the End of the Targets T.sub.1,
T.sub.2 : 50 mm
(5) Bias Power Source 41: Direct Current (0.about.75 Volt)
B. Operation Procedure
Argon gas was admitted into the vacuum vessel, until the pressure
was from 50 to 100 mm Torr. An alternating current of 50 Hz was
applied between the anode and cathode at 300 V for a period of 5
minutes, thereby inducing a glow discharge between the anode and
cathode and in the neighborhood of the surface of the base. Then
the argon gas pressure was decreased to 10 mm Torr, which was the
predetermined sputtering pressure. For a period of from 3 to 5
minutes, the base was shielded from the plasma gas by a shutter,
and then the shutter was opened to initiate the formation of a
perpendicular magnetic recording layer. The results are given in
the following table.
TABLE 9 ______________________________________ Preparation
Condition Bias Degree Voltage of Deposition Coercive of Sample
Direct Rate Force Curl Nos. Current (V) (.ANG./min) Hc(Oe) Kp (%)
______________________________________ 29 0 650 11 -20 (Control) 30
-50 650 10 -2 31 -75 650 10 +10
______________________________________
In the Permalloy layer of Sample No. 29 slight cracks were locally
detected. Sample Nos. 30 and 31 were free of cracks and exhibited a
small degree of curl.
* * * * *